In apparatus of this type, the flywheel is rotated by a rotary drive shaft to which it is connected by a rotary joint. Gyroscopic torques will tend to cause the flywheel to oscillate with two degrees of freedom about a central position in which the flywheel axis is aligned with the axis of the drive shaft.
When the apparatus is intended to operate as a rate gyro, flywheel return means are additionally provided suitable for exerting a precession torque on the flywheel tending to return it to its central position. The flywheel return means may be fixed electromagnets applying an attraction force on the rotor in response to a signal representative of the inclination (in two degrees of freedom) of the flywheel axis. A rate gyro of this type is described, for example, in published French patent application No. 2 394 060 in the name of the present assignees.
More precisely, the rotary joint is generally a Hooke's joint comprising four mutually perpendicular torsion bars interconnected by a central ring in such a manner as to enable the flywheel to tilt its axis in its two degrees of freedom for oscillation. The central ring of the Hooke's joint then acts as an intermediate inertia flywheel, and the value of its inertia is chosen so that, at the speed of rotation of the flywheel, it exactly compensates the stiffness of the torsion bars. The parameters of the suspension can thus be determined in such a manner that at its nominal speed of rotation, the flywheel is not subjected to any return force due to the suspension.
In this manner, a dynamic suspension stiffness compensating function is thus combined with the purely mechanical functions of radial support and rotary drive of the flywheel via the joint.
Such a structure may be referred to as a "tuned gyro" or as a "gyro having compensated elastic suspension" and has the advantage of being very simple, which means that it is cheap, easy to make and easy to adjust. Matching the compensation boils down in practice to adjusting the moment of inertia of the intermediate flywheel which can easily be done by conventional means for balancing rotating parts.
However, miniaturizing such tuned gyros is limited by the impossibility of reducing the inertias to very low values. Contradictory constraints are encountered concerning the need to provide adequate mechanical strength for the flywheel to hold together radially (generally speaking flywheels rotate at very high speed, e.g. about 400 revolutions per second), and the need to provide a sufficiently small stiffness constant since the stiffness constant needs to be reduced with reduced flywheel mass.
The degree to which rate gyros of this type can be miniaturized has thus been limited, up to now, by the need to find a compromise between these contraints of mechanical strength and of dynamic behaviour.
Preferred embodiments of the invention provide an elastically suspended gyroscopic structure which is free from this dilemna thus enabling the inertias to be made small enough for the size of the apparatus to be greatly reduced.